1. Field of the Invention
This invention generally relates to surface acoustic wave (SAW) filters, and, more specifically, to enhancing the performance of SAW filters.
2. Background
A conventional longitudinally-coupled SAW filter is illustrated in FIG. 1. The filter itself is identified with numeral 10. The filter is shown as it would typically be configured in operation, i.e., terminated with source and load impedances and coupled to a signal source. The signal source is identified with numeral 1; the source impedance, with numeral 2, and the load impedance, with numeral 7. The filter itself comprises two outer reflectors, identified respectively with numerals 3 and 6, and two interdigital transducers (IDTs), identified respectively with numerals 4 and 5, situated within the cavity 9 defined by the two outer reflectors 3 and 6. IDT 4, being coupled to the signal source 1 through the source impedance 2, functions as a source IDT, and IDT 5, being coupled to the load impedance 7, functions as a load IDT. In the conventional filter, the gap 8 between the source and load IDTs 4 and 5 is typically the spacing between IDT fingers plus roughly 0.7 Bragg lengths where a Bragg length is approximately defined as xc2xdxcex (xcex being the wavelength of the acoustic wave generated within the SAW filter). The Bragg length is a fixed value whereas the wavelength varies with frequency. The number of poles (natural frequencies) in the transfer function of the filter of FIG. 1 is limited to 3.
A second embodiment of the conventional SAW filter is illustrated in FIG. 2. As in FIG. 1, the filter, which is identified by numeral 19, is shown as it would typically be configured in operation, i.e., terminated with source and load impedances and coupled to a signal source. The signal source in this embodiment is identified with numeral 11; the source impedance, with numeral 12; and the load impedance, with numeral 18. The filter itself comprises two outer reflectors, identified respectively with numerals 13 and 17, with three IDTs, identified respectively with numerals 14, 15, and 16, in the cavity defined by the two outer reflectors 13 and 17. IDT 15, being coupled to signal source 11 through source impedance 12, functions a source IDT, while IDTs 14 and 16, being coupled to the load impedance 18, function as load IDTs. In this embodiment of the conventional SAW filter, the gaps 19 and 20 between the source and load IDTs are each typically the same as the first embodiment. Again, the number of poles in the transfer function of the filter of FIG. 2 is limited to 3.
As is known, each of the filters of FIGS. 1 and 2 are mounted on a piezo-electric substrate made of a material such as lithium tantalate (LiTaO3), lithium niobate (LiNbO3), or the like. FIGS. 1 and 2 each represent top views of the filters as they might appear on the surface of the substrate. In operation, when the A/C signal from the signal source is applied to the filter, an alternating electric field is created between the fingers of the IDTs making up the filter. This electric field causes the substrate to expand and contract at the frequency of the A/C signal applied by the signal source. The result is that a longitudinally-propagated acoustic wave is generated within the cavity defined by the outer two reflectors.
The frequency selectivity of a filter is defined by the number of poles in the transfer function of the filter. Generally speaking, the greater the number of poles, the greater the frequency selectivity of the filter. The situation is illustrated in FIG. 3, in which the frequency response of a filter with a small number of poles is identified with numeral 21, and the frequency response of a filter with a greater number of poles is identified with numeral 20. As can be seen, the frequency selectivity of the frequency response identified with numeral 20 (determined by the steepness of the slope of the sides) is greater than that of the frequency response identified with numeral 21.
As stated previously, the number of poles that is available in the filters of FIGS. 1 and 2 is three. If greater frequency selectivity is desired beyond that available through a three-pole filter, the conventional approach is to cascade one or more of the filters of FIGS. 1 and 2 in order to achieve a greater number of poles, and hence, greater frequency selectivity. The problem is that this results in excessive space being consumed by the filter, and also excessive insertion loss.
Consequently, there is a need for a SAW filter which is able to provide increased frequency selectivity while avoiding the excessive space consumed and insertion loss exhibited in the conventional approach.
In accordance with the purpose of the invention as broadly described herein, there is provided a surface acoustic wave filter comprising first and second reflectors defining a cavity, first and second interdigital transducers (IDTs) within the cavity and spaced by a gap, and at least one reflector within the gap. In one embodiment, the number of reflectors in the gap is equal to the desired number of filter polesxe2x88x923. The spacing between the reflectors in the gap, and between the IDTs and the reflectors in the gap; the number of fingers in the IDTs, and in the reflectors in the gap; the pitch of the IDTs and of the reflectors in the gap; and the aperture of the IDTs and of the reflectors in the gap provide the necessary degrees of freedom to achieve a desired frequency response shape, which is determined in part by the placement of the poles and zeroes of the filter transfer function. In one implementation, one or more of these parameters are selected to achieve an optimal filter in terms of one or more of the following criteria: 1) minimum ripple in the passband; 2) maximum attenuation in the stopband; 3) minimum insertion loss; 4) minimum sensitivity to parameter changes; and 5) maximum ease of buildability.
In a second embodiment, first, second, and third IDTs are placed within the cavity. The spacing between the first and second IDTs defines a first gap, whereas the spacing between the second and third IDTs defines a second gap. In this embodiment, at least one reflector is placed in the first gap, and at least one reflector is placed in the second gap. In this implementation, the filter exhibits mirror-image symmetry, such that the number of reflectors in the first gap is equal to the number of reflectors in the second gap. In this implementation example, the number of reflectors placed in each gap is equal to the number of desired polesxe2x88x923.
A surface acoustic wave filter system is also provided. In this system, an embodiment of a filter configured in accordance with the subject invention is terminated with a parallel load impedance and a series source impedance. The series source impedance is placed in series with an A/C signal source.
In one application, the filter system of the subject invention forms the RF filter of the front-end of a super-heterodyne transceiver.